The present invention is directed, in general, to surface acoustic wave (SAW) filters and, more specifically, to SAW filters having a diminished bandwidth and method of operating such filters to process signals.
Electronic signal processing by means of selective manipulation of a surface acoustic wave (SAW) on piezoelectric substrates typically uses thin-film transducer structures. These structures usually consist of an exactingly constructed opposing pair of parallel fingers sets that are interleaved, but do not touch. Each opposing pair of finger sets has an electrical terminal that allows an input AC signal to be coupled to one finger set and an output AC signal to be obtained from the other finger set. SAW filters act as an electrical-mechanical-electrical energy converter, the filtering being performed within the mechanical domain of the piezoelectric substrate. Application of an appropriate input electrical AC signal provides the stimulus to create a SAW that may typically be a Rayleigh wave with motion confined to about one acoustic wavelength under the free surface of the piezoelectric substrate. Alternatively, the acoustic excitation may be a xe2x80x9cleaky wavexe2x80x9d, which also finds application in modern radio frequency devices.
The electronics industry has applied such SAW signal processing to the design of analog electrical filters operating at selected frequencies in the range of about 10 megahertz to 2xc2xd gigahertz. SAW devices, operating in the Rayleigh wave mode, can generally be designed to provide quite complex signal processing functions within a single package that contains only a single piezoelectric substrate with superimposed thin-film interdigital transducers. For example, bandpass filters can be designed to achieve responses that would require several hundred inductors and capacitors in conventional LC filter designs.
SAW filter devices may be mass produced using semiconductor microfabrication techniques thereby providing outstanding reproducibility in performance from device to device. Since SAW filters may be implemented in small, rugged, light and power efficient modules, they find important applications in mobile, wireless and spaceborne communication systems.
SAW filter designers are under constant pressure to improve the insertion loss of filter networks, especially in applications involving low level signals at radio frequencies. Additionally, SAW filter designers must combine a suite of SAW transducers to provide a required bandpass shape. Shaping the passband response often requires using a collection of many different SAW transducers. Each of the SAW transducers must typically be designed individually wherein its resonant frequency, and therefore its interdigital structure and associated individual response width, is chosen to meet a particular overall network passband requirement. The inability to easily tailor individual SAW resonator response widths (coupling) for a chosen resonant frequency makes the collective passband shaping more difficult and increases the cost of the solution.
Accordingly, what is needed in the art is a simple and cost effective way to judiciously narrow the bandwidth of a SAW filter resonator.
To address the above-discussed deficiencies of the prior art, the present invention provides an element of a SAW filter network, a method of filtering a signal and a SAW filter network incorporating the element or the method. In one embodiment, the element includes: (1) a SAW resonator having a nominal bandwidth and (2) an extrinsic capacitor coupled in parallel with the SAW resonator, the extrinsic capacitor interacting with the SAW resonator to cause an anti-resonance frequency of the element to move toward a resonance frequency thereof and thereby decrease an overall operating bandwidth of the element.
The present invention introduces the broad concept of employing an extrinsic capacitor (a capacitor outside of the SAW resonator itself) to modify the anti-resonance frequency of the SAW resonator. The element (a combination of the SAW resonator and the extrinsic capacitor) is thereby given a diminished operating bandwidth (passband or notch). A diminished operating bandwidth is particularly advantageous in low cost applications, wherein downstream signal processing benefits from improved filtering.
In one embodiment of the present invention, the SAW resonator and the extrinsic capacitor share common terminals. In an embodiment to be illustrated and described, the SAW resonator and extrinsic capacitor are formed between a pair of parallel terminals on a common piezoelectric substrate.
In one embodiment of the present invention, the SAW filter element is a first SAW resonator and the element further includes a second SAW resonator non-transversely coupled in series with the first SAW resonator. In a related embodiment, the SAW resonator is a first SAW resonator and the SAW filter element further includes a second SAW resonator non-transversely coupled in series with the first SAW resonator and in parallel with the extrinsic capacitor. In an embodiment to be illustrated and described, certain elements of a SAW filter network include first and second SAW resonators non-transversely coupled in series and a single extrinsic capacitor coupled in parallel thereto. The extrinsic capacitor causes the anti-resonance frequencies of both the first and second SAW resonators to approach the resonance frequencies thereof and thereby narrow the bandwidth of the element as a whole.
In one embodiment of the present invention, the extrinsic capacitor has an intrinsic resonant frequency greater than the resonance frequency. Alternatively, the extrinsic capacitor has an intrinsic resonant frequency less than the resonance frequency.
In one embodiment of the present invention, the SAW resonator comprises an interdigitated central region and opposing shorted end regions. In a manner that is known to those skilled in the pertinent art, the interdigitated central region supports a SAW, and the shorted end regions reflect and contain it.
In one embodiment of the present invention, the SAW filter network has a resonance frequency between 900 and 920 megahertz. Although those skilled in the pertinent art will understand that such frequency is particularly advantageous for portable telephone applications, the present invention is not limited to a particular band or bandwidth.
The foregoing has outlined, rather broadly, preferred and alternative features of the present invention so that those skilled in the art may better understand the detailed description of the invention that follows. Additional features of the invention will be described hereinafter that form the subject of the claims of the invention. Those skilled in the art should appreciate that they can readily use the disclosed conception and specific embodiment as a basis for designing or modifying other structures for carrying out the same purposes of the present invention. Those skilled in the art should also realize that such equivalent constructions do not depart from the spirit and scope of the invention in its broadest form.